"We're not over with this bull market," Siegel notes. "Our economy is the best of the three major engines in the world, and that's why I have faith in U.S. stocks."

He expects that Friday's jobs report will show non-farm payrolls rose more than 200,000 in August. That would mark the seventh straight month of gains above 200,000. "This looks good for earnings," Siegel earnings. "We're going to get about $120 on the S&P [earnings]. I like that."

He doesn't anticipates sharp rises in interest rates that would hurt stocks.

"We've lived long enough to see Treasury rates at 5, 6, 7, 8 percent. I'm not scared of 3 percent. I'm not scared of 3.5 percent."

James Paulsen, chief investment strategist at Wells Capital Management, thinks the five-year bull market may have another five years to go.

But "I am a little worried," he tells The Wall Street Journal. Stocks may plummet 15 percent before the end of the year, Paulsen notes. "Bull markets aren't typically straight-line events."